A belt press, such as described in the commonly assigned and copending application Ser. No. 927,924 (now U.S. Pat. No. 4,187,776 has a pair of platens which are pressed against opposite sides of a conveyor or transport belt, either to repair same, or in the manufacturing or joining together of the ends of such a belt. Such platens must be heated to a treatment temperature, which is in the case of a rubber-type belt the vulcanization temperature of the rubber, and in the case of a synthetic-resin belt the hardening or softening temperature for the resin.
Such a press platen is normally flat and rectangular, trapezoidal, parallelogrammatic or rhombic. Each platen normally has a stiffening frame of profiled bars provided if necessary with a diagonal stiffening member. The platen is heated by means of resistance wires embedded in a heater plate that is laminated with a press plate and forms therewith the press platen.
It is essential that the press platen be uniformly heated. The heat-treating operation carried out by a press platen in a belt press is relatively delicate, with uniform heating of the belt being treated being absolutely essential.
The known press platens are constructed with extreme care in order to achieve this uniformity. The distribution of the heaters is carefully calculated, and frequently extremely complex control arrangements are provided to ensure uniform heating. It is also necessary to provide a complex energizing circuit, as the relationship of the temperature of the heater to its resistance varies with temperature in a nonlinear and often relatively complex manner. This variation often requires the use of feedback-employing control circuits which operate relatively complex variable power supplies.